GPS assistance message and data issue identifiers for transmission to GPS enabled mobile stations in cellular communications networks and methods therefore. The GPS data issue identifiers indicate whether GPS data, for example corresponding ephemeris and almanac data, stored at the mobile station requires updating. In the exemplary 3.sup.rd generation (W-CDMA/UMTS) architecture, the GPS assistance message is a System Information Block (SIB), and the GPS ephemeris data identifier and corresponding satellite identifier is encoded in a value tag included in a Master Information Block (MIB).